This invention relates generally to emission control for internal combustion engines, and more particularly to a control system for diesel engines that provides integrated control of engine and post-combustion emission control devices, the latter including both DPF and LNT devices.
New emission limits call for major reductions in oxides of nitrogen (NOx) and particulate matter (PM) emissions from diesel engines. Engine manufacturers have developed systems for exhaust gas recirculation (EGR), and diesel particulate filters (DPF) to achieve low NOx/PM emissions. The use of DPFs has been found to reduce PM below the stringent requirements of the new emission standards proposed for the 2005 through 2010 time frame. However, NOx emissions are still six to eight times higher than the proposed standards for model year 2007.
To achieve the target NOx emission levels, new post-combustion devices are being experimented with. These devices include selective catalytic reduction (SCR) using urea or ammonia as reductant, and lean NOx traps (LNT) (otherwise known as NOx adsorbers) using hydrocarbon as reductant. Although SCR systems have been used in stationary applications for several years they are now being developed for the mobile fleet with good success. However, greater NOx reduction than SCRs can deliver is still hoped for and the LNT system promises to achieve the desired NOx conversion efficiency.
The invention is directed to methods and systems for controlling diesel engine emissions. In all embodiments, the diesel engine""s exhaust system has at least a diesel particulate filter (DPF) and a lean NOx trap (LNT). The DPF may be one of two types: a first type that uses a catalyzed soot filter or a second type that uses a continuously regenerated trap. In either case, the LNT is monitored to determine the need for regeneration or desulfurization. In the former case, the catalyzed soot filter is also monitored to determine the need for regeneration. A fuel injector is used to inject fuel upstream of the LNT under certain conditions, and a bypass may be used to bypass exhaust upstream of the LNT to provide richer or hotter exhaust to the LNT. The sensor outputs and controls for providing the appropriate heat or fuel mix for regeneration and desulfurization may be controlled with enhancements to existing engine control circuitry.
Features of the invention include the capability of integration of engine controls with control of post combustion emission control devices. Intake throttling, existing EGR systems, and post-combustion injection can be used, alternatively or in combination, for the purpose of regenerating the LNT. Existing EGR can be used to reduce NOx, without adversely affecting the ability to maintain low PM emissions. A portion of the exhaust can be diverted to assist in creating a stoichiometric air-to-fuel ratio at the inlet of the LNT to facilitate regeneration. Overall, the system performs all of the above while not affecting drivability.